Major spoilers from the novel tho..

Spoilers from the novel so they might not be accurate.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

In the past timeline when Fl died the Ml whom she fought with and got to know on the battlefield was in despair, a figur he truted offer him the Demon Kings heart to eat. ( The Demon king had power over time and space). so he could return FL to her past self and have a happy ending.

The Black mana that becomes active in Ml after the visit to the anchient ruins is the Demon kings power awakening. = Ml is the next rewival of the Demon king now..

Major Spoilers below.

The figur that offer the Ml the Demon kings heart to eat... was a high ranking priest!.
The reason the Holy statues place though out the kindom are "mailfunctining" at keeping the plague away is because the holy magic pured into them wasnt holy magic, but dark mana!.

The Church is behind most of the bad shit happening and its all because they fear the that all the peace has caused the citicens to lose faith. SO the higher ups in the church desided to have a revelatinon of the "revival" of the Demon King to reignite the peoples faith in the church.
But in reality the church plotted it all! Its the same for the mutated "demons" that attacked Fl in earlier chapters (the giant albino praying-mantices), they were artificialy made to cause chaos and "kill or injure" the Ml's love to get him focused on destroying the Demons and the reviving demon king.

The ex and cousin get screwed. They get blackmaild by a priest i think (forching them to try and finish the magic experiment that almost killed Fl in her last life).
Ex fuckes up and ends up in prison where cousin convinces him to do the experiment to escape! (Cousing has come to hate the fl's ex). The experiment fail, ex dying as a result.
Cousin inform Fl of this, expecting her to be happy about it (trying to win fawor agaain). Fl is not pleased and cousin gets chased out. The Blackmailer catches up to cousin and forces him to also do the magic experiemnt. ( duno if he survives, but if he does he would be worse of than old Fl).

Not all priests and higherups in the church are in on this tho. This is proven when the dark man in Ml starts to fully awaken and he starts to lose himself. Most high priest and elder priests beg Fl to end the Ml's life befor the Demon King fully revives and dooms the world, but some say to "capture him and seal him away"(monstly bad guys). Fl remebers her promis to protect Ml and comes up with an alternate plan, she consults the Emporer, Ml's knight, the head butlor and some priests that trusts her because of her focus on the plague cure. When the higher ranking priests confront Ml ( almost fully awakend) the binding spell they prepared fails, the priest betting Fl to kill Ml begges her to finish it because she is the only one that can. She asks why he must die, and who cant she try an alternate solution "why must he be sacrficed?". The Elder priest hesistates to answer and the corrupt priests com up with half hearted arguments. Elder priest allows her to try, and Fl uses her holy spirit to enter Ml's mind to clear the corruption.

IN Ml's mind Fl sees his past childhood, and all the times he has lost faith in humanity, she also finds memories form the past traped in the dark mana. She clears the mana and gets him free in his mind, only for a voice to talk to her and ask her why she would save him and wouldnt all bad things end and so on. Fl answers back that she promised to protect Ml and will stick to it no mater what, holy ligh sorounds them.

Fl faces off agaist the Demon king ( unsure if its still in ml's head, if it has taken ml's body or as left ml's body and is now physical) The Demon King asks Fl questsion she answers every question like.

DK: Do you still have faith in mankind?
Fl: Yes.
Dk: why? look at alll the bad ppl, (ex, cousin, nobels, priests)
Fl: Because not all humans are bad, using Ml as an example ( because of his sacrifice in the past)
Dk: Who do you think the true monster is me or humanity?
Fl: .. There are bad people but also good, look at redhaired paladin who hated summoners and is now on good terms with her and even supports his sister studying summoning. People can change. (unsure of her full answer)
Dk: what if they dont change?
Fl: we can try to teach them, inforce changes or reform,

Dk: I wish someone like you existed back when i was human, before people betraid me and looked at me with fear. (FL remembering ppl's looks when she was disfigured)

Dk: can you realy save Ml?
Fl: i will do all in my power trying, he saved me and gave me a new life so i will use this life to save him and ease his burden. ( yada yada yada,, speach)

DK is satisfied and diserpears and Ml has holy power and can wield the holy sword again.

They become a official couple and Fl gets pregnant.